This invention relates generally to air conditioning evaporators and, more particularly, to evaporators with two-phase refrigerant flow distribution.
In the cooling phase of a refrigeration system the heat exchanger referred to as an evaporator receives liquid refrigerant by way of an expansion valve, with the expanding refrigerant then tending to cool the liquid being separately circulated through the evaporator. The fluid to be cooled carries the heat load which the air conditioner is designed to cool, with the evaporator then transferring heat from the heat load to the liquid refrigerant.
The fluid to be cooled may flow through the evaporator by way of a bundle of pipes having heat conductive walls, with the liquid refrigerant being distributed on the outer surface of the pipes for the purpose of effecting the heat transfer function.
One approach for distributing the refrigerant to the outer surface of the pipe bundle is that of a falling film evaporator wherein the liquid refrigerant is sprayed horizontally by a sprayer so that it contacts the outside of the pipe bundle. The refrigerant then flows by gravity from the top horizontal pipes to the bottom horizontal pipes, while cooling the liquid flowing within the pipes during the process.
One problem with such an approach is that the sprayed liquid refrigerant tends to splash off the surface of the pipes to thereby reduce the intimacy of contact between the refrigerant and heat exchange surface. Further, it is difficult to control the axial distribution of liquid refrigerant along the length of the tubes. This is especially true when considering that, as the liquid refrigerant is discharged from the expansion valve, a portion of it will be in a liquid/vapor (two-phase) state, and as the refrigerant flow depletes as a result of being distributed through a perforated pipe or the like, the flow pattern in the distribution conduit can become stratified. The result is a maldistrubtion of refrigerant over the heat exchanger pipes.
One common approach to solving the problem is to use a liquid-vapor separator to separate the liquid and vapor phases coming from the expansion valve. This can be accomplished by either an internal or external liquid-vapor separator. However, in either case such an addition represents a substantial increase in cost, weight and manufacturing complexity.
It is therefore an object of the present invention to provide an improved method and apparatus for refrigerant distribution in a falling film evaporator.
Another object of the present invention is the provision for effectively distributing two-phase refrigerant in a falling film evaporator.
Yet another object of the present invention is the provision for an improved method and apparatus for distributing two-phase flow in a uniform manner over the heat transfer tubes of an evaporator.
Still another object of the present invention is the provision for a falling film evaporator that is economical to manufacture and effective and efficient in use.
These objects and other features and advantages become readily apparent upon reference to the following descriptions when taken in conjunction with appended drawings.
Briefly, in accordance with one aspect of the invention, a header which receives two-phase refrigerant from the expansion valve is provided with a nozzle which provides a pressure drop and an increase in velocity to propel the two-phase refrigerant flow into the header. The structure of the header thus provides a closed circuit such that the refrigerant makes a complete cycle through the header to return to the nozzle. In this way, the nozzle provides for a continuous flow of the two-phase refrigerant mixture through the header to thereby ensure a uniform distribution to the individual subheaders that are fluidly interconnected to the header.
By yet another aspect of the invention, the header is a two-pass structure interconnected by a return bend, with the first pass having openings that are fluidly connected to subheaders, and the second pass is simply provided to return the flow from the return bend to the nozzle at the other, upstream, end of the first pass.
In accordance with another aspect of the invention, there are provided a plurality of subheaders with each having a similar structure as that of the header, including nozzle, a first pass, a return bend and a second pass. In addition, the first pass has at its lower surface, a plurality of opening through which liquid refrigerant can flow downwardly to engage the upper surface of the second pass. The refrigerant then is uniformly distributed over the upper surface of the second pass, running down over the sides and is then uniformly distributed over the heat transfer tubes below.
In the drawings as hereinafter described, a preferred embodiment is depicted; however various other modifications and alternate constructions can be made thereto without departing from the true spirt and scope of the invention.